System and method for non-destructive measurement of dynamic coercivity effects

ABSTRACT

A method for non-destructive measurement of dynamic coercivity is disclosed. In this method, magnetic media is DC erased by applying a forward DC magnetic field to the magnetic media such that the magnetic moments in the magnetic media are substantially aligned. A specified number of reversed magnetic field pulses are then applied to the magnetic media in a direction opposite to the forward DC magnetic field, wherein the intensity of the reversed magnetic field pulses is less than the remanent coercivity of the magnetic media. The broadband medium noise of the magnetic media is measured. The intensity of the reversed magnetic field pulses is then repeatedly and incrementally increased and applied to the write head for the specified number of pulses, the intensity of the reversed magnetic field pulses eventually exceeding the remanent coercivity of the magnetic media. For each intensity level of the reversed magnetic field pulses, the broadband medium noise is again measured. The coercivity of the magnetic media is derived from the reversed magnetic field pulse intensity at which the broadband medium noise is a maximum. The entire process may be repeated for different numbers of pulses in order to measure dynamic coercivity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/179,297,filed Oct. 27, 1998 now U.S. Pat. No. 6,304,081, which application(s)are incorporated herein by reference.

Embodiments of this invention relate to Provisional Application Ser. No.60/069,197, filed Dec. 11, 1997. The contents of that application areincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of this invention relate generally to the measurement ofdynamic coercivity effects in magnetic media, and in particularembodiments to methods for non-destructive measurement of dynamiccoercivity effects in magnetic media, and systems incorporating thesame.

2. Description of Related Art

Modern computers require media in which digital data can be quicklystored and retrieved. Magnetizable (hard) layers on disks have proven tobe a reliable media for fast and accurate data storage and retrieval.Disk drives that read data from and write data to hard disks have thusbecome popular components of computer systems.

As the recording media industry strives for increasingly smaller diskdrives with increasingly larger storage capability, the areal density(the number of bits, or magnetic flux reversals, per inch) of magneticdisks continues to increase. In order to maintain a sufficientsignal-to-noise ratio, the number of magnetizable units in each storedbit must be kept above a certain minimum value. To reduce bit size whilemaintaining a certain number of magnetizable units in each bit, the sizeof the magnetizable units (the particle or crystal grain size) musttherefore be correspondingly decreased.

However, very small particles may be magnetically unstable due to theeffect of thermal agitation (onset of superparamagnetism). Typically, anenergy barrier inherent in a magnetized particle prevents the particlefrom reversing its magnetization, but as particle size decreases, thisenergy barrier also decreases. As the energy barrier becomesincreasingly small, the likelihood that a particle will spontaneouslyreverse its magnetization due to thermal agitation over a given timeperiod increases. This state, known as superparamagnetism, ischaracterized by low coercivity (the ability of a particle to resistmagnetic change) and low remanent coercivity (the magnetic fieldrequired to reduce the magnetization of the particle to zero), andresults in a particle with high magnetic instability.

Particle coercivity is also dynamic. Generally, particle coercivity ishigh when measured over short periods of time, but is low when measuredover extended periods. Low coercivity over extended periods of time maycause the magnetic media to demagnetize. In addition, as thesuperparamagnetic regime is approached, the time-dependence ofcoercivity increases. Thus, the related factors of small particle size,low coercivity, and the onset of superparamagnetism cause recorded bitsto become unstable, increasing the likelihood that stored informationwill be lost.

The time-dependence of coercivity is particularly relevant to magneticrecording media because of the need to support both short-term writingcapability and long-term data storage stability. An apparent solution tothe problem of magnetic instability would be to utilize a magneticmaterial with a larger anisotropy constant to increase the “storage” or“long-term” coercivity until the desired stability of the magneticallystored information is achieved. However, this may also cause asignificant increase in short-term coercivity and consequent writingdifficulties. A balance between short-term and long-term particlecoercivity must be achieved to produce both reliable data writing andstable data storage. The measurement of dynamic (time-dependent)coercivity effects in magnetic media is therefore an importantcapability in the design and development of magnetic recording media.

Conventional methods for measuring dynamic coercivity effects haveemployed destructive techniques for making the measurement. In thesemethods, test coupons or samples are separated from the magnetic mediaand placed in a test fixture for analysis by a magnetometer. Forexample, the article “Reptation effects in particulate systems. II.Experimental studies” by Lewis, et al., published in the Journal ofApplied Physics, Vol. 73, No. 10, May 15, 1993, discusses themeasurement of the time-dependence of coercivity using a sample ofmagnetic film and an alternating gradient force magnetometer (AGFM). Inthis approach, a sample must first be taken from the magnetic media andplaced in a separate tester. The necessity of taking a sample physicallydestroys the magnetic media so that no further measurements can be made.

SUMMARY OF THE DISCLOSURE

Therefore, it is an object of embodiments of the invention to provide asystem and method for non-destructive measurement of dynamic coercivityeffects in magnetic media such that subsequent measurements may beobtained from the magnetic media.

It is a further object of embodiments of the invention to provide asystem and method for measurement of dynamic coercivity effects inmagnetic media on a standard magnetic media parametric tester such as aspinstand tester so that separate testing devices such as magnetometersneed not be used.

These and other objects are accomplished according to a method fornondestructive measurement of dynamic coercivity. In this method,magnetic media is DC erased by applying a forward DC magnetic field tothe magnetic media such that the magnetic moments in the magnetic mediaare substantially aligned. A specified number of reversed magnetic fieldpulses are then applied to the magnetic media in a direction opposite tothe forward DC magnetic field, wherein the intensity of the reversedmagnetic field pulses is less than the remanent coercivity of themagnetic media. The broadband medium noise of the magnetic media ismeasured. The intensity of the reversed magnetic field pulses is thenrepeatedly and incrementally increased and applied to the write head forthe specified number of pulses, the intensity of the reversed magneticfield pulses eventually exceeding the remanent coercivity of themagnetic media. For each intensity level of the reversed magnetic fieldpulses, the broadband medium noise is again measured. The coercivity ofthe magnetic media is derived from the reversed magnetic field pulseintensity at which the broadband medium noise is a maximum. The entireprocess may be repeated for different numbers of pulses in order tomeasure dynamic coercivity.

These and other objects, features, and advantages of embodiments of theinvention will be apparent to those skilled in the art from thefollowing detailed description of embodiments of the invention, whenread with the drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a system for non-destructivemeasurement of dynamic coercivity effects in magnetic media according toan embodiment of the present invention.

FIG. 2 illustrates the DC erasing of a track according to an embodimentof the invention.

FIG. 3 illustrates partial reverse DC erasing of a track according to anembodiment of the invention.

FIG. 4 is a symbolic representation of reptation according to anembodiment of the invention.

FIG. 5 illustrates the reading of an information signal from a partiallyreverse DC-erased track using a magneto-resistive read head according toan embodiment of the invention.

FIG. 6 is graph of reverse DC erase noise voltage curves versus writecurrent for differing numbers of media rotations according to anembodiment of the invention.

FIG. 7 is a graph of remanent coercivity versus reverse magnetic fieldpulsewidth according to an embodiment of the invention.

FIG. 8 is a graph showing a comparision of remanent coercivity versusreverse magnetic field pulsewidth obtained from a rotating diskmagnetometer (RDM), an AGFM, and an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following description of preferred embodiments, reference is madeto the accompanying drawings which form a part hereof, and in which isshown by way of illustration specific embodiments in which the inventionmay be practiced. It is to be understood that other embodiments may beutilized and structural changes may be made without departing from thescope of the preferred embodiments of the present invention.

As the recording media industry strives for increasingly smaller diskdrives with increasingly larger storage capability, the areal density(the number of bits, or magnetic flux reversals, per inch) of magneticdisks continues to increase. In order to maintain a sufficientsignal-to-noise ratio, the number of magnetizable units in each storedbit must be kept above a certain minimum value. To reduce bit size whilemaintaining a certain number of magnetizable units in each bit, the sizeof the magnetizable units (the particle or crystal grain size) musttherefore be correspondingly decreased.

However, very small magnetic particles are characterized by lowcoercivity (the ability of a particle to resist magnetic change) and lowremanent coercivity (the magnetic field required to reduce themagnetization of the particle to zero), and as a result, tend to bemagnetically unstable.

Particle coercivity, and hence magnetic stability, is also dynamic.Generally, particle coereivity is high when measured over short periodsof time, but is low when measured over extended periods. A balancebetween short-term and long-term particle coercivity must be achieved toproduce both reliable data writing and stable data storage. Themeasurement of dynamic (time-dependent) coercivity effects in magneticmedia is therefore an important capability in the design and developmentof magnetic recording media.

FIG. 1 illustrates an example of a coercivity measurement system 10 foruse in measuring dynamic coercivity according to an embodiment of theinvention. The coercivity measurement system 10 comprises a computercontrol panel 12, a spin stand tester 14, and a frequency analysisdevice 16. The spin stand tester 14 comprises a computer 18, aread/write controller 20, a preamplifier 22, a read/write head assembly24, a head positioner 42 coupled to the read/write head assembly 24, aspindle motor 40, a spindle 26 rotatably coupled to the spindle motor40, and a disk 36 coupled to the spindle 26.

An operator (not shown in FIG. 1) controls the coercivity measurementsystem 10 by entering commands into the computer control panel 12, whichcommunicates with the spin stand tester 14 through computer controlsignals 28. Responsive to the computer control signals 28, the computer18 communicates with the read/write controller 20 through read/writecontrol signals 30, communicates with the head positioner 42 throughhead positioner control signals 44, communicates with the spindle motor40 through spindle motor control signals 52, and communicates with thefrequency analysis device 16 through frequency analysis device controlsignals 48. Responsive to the read/write control signals 30, theread/write controller 20 communicates with the preamplifier 22 throughpreamplifier control signals 32. Responsive to the preamplifier controlsignals 32, the preamplifier may source write current 38 to theread/write head assembly 24 through conductors 34, sense voltages fromthe read/write head assembly 24 across the conductors 34, or communicatewith the frequency analysis device 16 through information signals 50.Responsive to the head positioner control signals 44, the headpositioner 42 may move the read/write head assembly 24 radially withrespect to the disk 36. Responsive to the spindle motor control signals52, the spindle motor 40 may rotate the disk 36 at a variable number ofrevolutions per minute (RPM). Responsive to frequency analysis devicecontrol signals 48, the frequency analysis device 16 may communicatedata to the computer 18 through frequency analysis device data signals46.

To measure dynamic coercivity, an operator uses the computer controlpanel 12 to initiate a program stored in computer 18. In alternativeembodiments of the invention, the program may be stored in hardware,software, or firmware of the computer 18 or a peripheral (not shown inFIG. 1).

The idea of the measurement technique is to use the reverse DC erasednoise (broadband medium noise) as an indicator for coercivity. Aftermagnetizing a band of overlapping tracks in one direction such that asubstantial number of magnetic moments are aligned, a DC magnetic fieldin the reverse direction is applied for a specified number of pulses.The broadband medium noise is then derived from a signal read from themagnetic media. As a function of the write current applied in thereverse direction and the number of applied magnetic field pulses, thebroadband medium noise of the overwritten tracks is at a maximum wherethe applied DC magnetic field in the reverse direction is equivalent tothe coercivity of the magnetic media.

As illustrated in FIG. 2, the program may first cause a DC-erase to beperformed such that all magnetic moments in the test area of the disk 36are oriented in the same direction. To accomplish this, under programcontrol the preamplifier 22 (see FIG. 1) sources a constant writecurrent 38 through a plurality of conductors 34 and into the read/writehead assembly 24. The read/write head assembly 24 includes a core 54shaped to form a gap 56, and a coil 58 wrapped about the core 54 andcoupled to the conductors 34. The constant write current 38 flowingthrough the coil 58 induces a constant magnetic flux (not shown in FIG.2) to form in the core 54 and fringe across the gap 56, orientingsubstantially all magnetic moments under the gap 56 in the samedirection and creating a track of DC-erased magnetic moments 60 as thedisk 36 under test spins in the direction indicated by the arrow 62. Theprogram then directs the head positioner 42 (see FIG. 1) to radiallyreposition the read/write head assembly 24 over adjacent areas of thedisk 36, and the erase process is repeated until a sufficiently widearea of the disk 36 has been DC-erased. In alternative embodiments ofthe present invention, the magnetic media need not be rotatable, and maybe DC-erased by simply sourcing constant write current 38 through aplurality of conductors 34 and into a read/write head assembly 24positioned over stationary magnetic media, or by positioning a permanentmagnet over the magnetic media.

Next, as illustrated in FIG. 3, the program causes the reverseDC-erasing of the disk 36 such that some of the magnetic moments reversetheir direction. The process to accomplish this is very similar to theprocess for DC-erasing, except that the direction of the write current38 is reversed. By reversing the direction of the write current 38 asthe disk 36 spins in the direction indicated by the arrow 62, the disk36 is subjected to reversed magnetic field pulses oriented opposite tothe DC-erased magnetic moments, and reversed orientation magneticmoments 64 may appear, overwriting the homogeneous orientation ofDC-erased magnetic moments 60. The number of reversed orientationmagnetic moments 64 is dependent on the magnitude of the reversed writecurrent 38 (and hence the intensity of the reversed magnetic fieldpulses)-the higher the current, the more reversed orientation magneticmoments 64 will appear. For example, if the reversed write current 38produces reversed magnetic field pulses with an intensity well in excessof the coercivity of the magnetic media, a substantial number ofmagnetic moments will be reversed. If the reversed magnetic field pulseintensity is substantially below the coercivity of the magnetic media,few magnetic moments will be reversed. If the reversed magnetic fieldpulse intensity approaches the coercivity of the magnetic disk 36, eachtime the read/write head assembly 24 passes over a particular area onthe disk 36, more reversed orientation magnetic moments 64 will appearin that area of the disk 36.

The duration of the reversed magnetic field pulse is approximately givenby t_(p)≈g/v, where g is the gap length 56 and v is the linear velocityof the disk 36 as it spins in the direction 62. Pulse duration can bevaried by changing the linear velocity or changing the gap length.Another way to change the effective pulse duration is to subject themagnetic media to many of these field pulses rather than to one. In thatcase, t_(p,n)=n*t_(p,1)≈n*g/v, where n is the number of passes of theread/write head assembly 24. As illustrated in FIG. 4, where M is theintensity of magnetization of the magnetic media and H is the appliedmagnetic field intensity, the application of several field pulsesresults in cycling through minor loops, each loop cycling between noapplied magnetic field (H=0) and an applied magnetic field atapproximately the remanent coercivity of the material (H_(r)). Theprocess sketched in FIG. 4 is called “reptation.” The repeatedapplication of n pulses can be approximated as one pulse ofcorrespondingly longer duration, which allows time-dependent measurementof the magnetic material's coercivity. In preferred embodiments of thepresent invention, the linear velocity is approximately 45 m/s (a largeradius disk 36 spinning at approximately 10000 RPM), which results in apulse length of about 10 ns, and the read/write head assembly 24 has agap 56 of 500 nm and is operated at zero skew.

It should be noted that although in preferred embodiments of the presentinvention reptation is typically achieved by rotating magnetic mediapast a DC magnetic field, in alternative embodiments a pulsed magneticfield on stationary magnetic media may also be used.

The coercivity is measured by reading and deriving the broadband mediumnoise from the magnetic media after a specified number of disk rotationsor pulses have been applied. Referring to FIG. 5, to read and computethe broadband medium noise, a read head is positioned over the reverseDC-erased area of the disk 36 by the head positioner 42 (see FIG. 1)under program control. In preferred embodiments of the invention, theread/write head assembly 24 includes a magneto-resistive (MR) read head66. The MR read head 66 is comprised of a conductive element whoseresistivity varies as a function of an applied magnetic field. In an MRhead, a constant current is sourced through the element and a biasmagnetic field is applied in an orientation approximately 45° from thedirection of the current, and as the head is placed in close proximitywith magnetized areas on the disk 36, the applied magnetic field fromthe magnetic moments on the disk 36 causes the resistance of theelement, and hence the voltage across it, to change. If the net magneticfield (the bias magnetic field plus the applied magnetic field from thedisk 36) is oriented parallel to the current, the resistance and voltageacross the element increases. If the net magnetic field is orientedperpendicular to the current flow, the resistance and voltage across theelement decreases. Thus, by sensing the voltage across the element,stored data in the form of oriented magnetic moments can be read. Inalternative embodiments of the invention, the core 54 of the read/writehead assembly 24 (see FIG. 2) also serves as the read head. In suchembodiments, magnetic fields from the magnetic moments form in the core,and a voltage is induced in the coil 58. Again, by sensing the voltageacross the coil, stored data in the form of oriented magnetic momentscan be read.

Referring again to FIG. 5, when the MR head 66, under program control,is positioned over a partially reverse DC-erased area on the disk 36,the MR head 66 senses a voltage and communicates this voltage to thepreamplifier 22 through conductors 34 (see FIG. 1). The preamplifier 22transforms the voltage reading into an information signal 50, andcommunicates the information signal 50 to the frequency analysis device16. The frequency analysis device 16 performs a Fourier transform on theinformation signal 50 and computes and stores the resultant Fourierspectrum. It should be noted that in alternative embodiments of theinvention, the frequency analysis device 16 may be any device, system,or analog filter that can perform a Fourier transform or a fast Fouriertransform on the information signal 50 or derive all or part of thefrequency spectrum from the information signal 50. The data pointscomprising the frequency spectrum are then communicated to the computer18 through frequency analysis device data signals 46.

The resultant data points represent the noise power spectrum. Abroadband media noise calculation is then made by integrating the noisepower spectrum over a predetermined frequency range. In preferredembodiments of the present invention, the noise is integrated between 5and 160 kfci, and the electronic and head noise is removed before theintegration. Measurements have confirmed that the peak of the noiseoccurs at the coercivity of the magnetic material (see “DC ModulationNoise and Demagnetizing Fields in Thin Metallic Media” by Tarnopolsky etal., published in IEEE Transactions in Magnetics, Vol. 25, No. 4, July1989, incorporated herein by reference).

It should be noted that the interpretation of measured data must beviewed with caution, because there is a demagnetization field associatedwith the pseudo-transition occurring when the reversed directionmagnetic field switches the medium magnetization from the saturatedremanent state to M=0. This demagnetization field emanating from themagnetic media shields the reversed direction magnetic field such thatthe actual magnetic field seen by the medium is somewhat lower than theapplied reversed direction magnetic field itself. Compensation for thedemagnetizing field is discussed in “Modulation Noise and DemagnetizingFields in Thin Metallic Media” by Tarnopolsky, et al., published in IEEETransactions on Magnetics, Vol. 25, No. 4, July 1989, and isincorporated herein by reference.

The basic steps of DC erasing an area on magnetic media, applying areversed DC current (and hence a reversed magnetic field) for aspecified number of rotations or pulses, and measuring the broadbandmedium noise, is repeated for various currents and various rotations orpulses. As illustrated in FIG. 6, a family of curves of write current(I_(w)) versus broadband media noise (Nm) in arbitrary units (a.u.) maybe obtained for different numbers of revolutions or pulses. The numberof pulses in FIG. 6 was increased according to the formula 3^(m), wherem varied from zero to nine. However, in alternative embodiments of thepresent invention, any number of pulses may be utilized to characterizethe dynamic coercivity of the magnetic media under test. Note that asthe effective pulse width of the reversed magnetic field increases(increased number of rotations), the write current (and hence theapplied reverse magnetic field) at which the broadband media noise is ata maximum decreases. Because the applied reverse magnetic field thatmaximizes media noise is approximately equivalent to the coercivity ofthe magnetic media, FIG. 6 illustrates the dynamic or time-dependentnature of coercivity, wherein coercivity decreases as the duration of anapplied magnetic field pulse increases.

Because the write current is proportional to the applied magnetic field,the currents can be converted back to magnetic fields. The currents canbe calibrated by measuring the coercivity with a non-destructiverotating disk magnetometer (RDM). The conversion process is described in“Spatially resolved, in-situ measurements of the coercive force of thinfilm media” by R. D. Fisher and J. P. Pressesky, published in IEEETransactions on Magnetics, Vol. 25, pp. 5547-49, May 1994, and isincorporated herein by reference. The result of this conversion is shownin FIG. 7, where Hr is the remanent coercivity in Oerstads (Oe). FIG. 7again confirms that coercivity decreases as the duration of an appliedmagnetic field pulse increases.

FIG. 8 compares remanent coercivity versus reversed magnetic fieldpulsewidth data for embodiments of the present invention (blackcircles), a rotating disk magnetometer (white circle), and an AGFM(triangles). For the AGFM measurement, the demagnetizing field for eachfield point was applied for 0.1, 1, 10, 100, and 1000 seconds,respectively, and a very large sweep rate was utilized. FIG. 8 alsoshows that the range in which coercivity changes logarithmically withtime is very large, with a slight deviation for very short times.

Therefore, according to the foregoing description, preferred embodimentsof the present invention provide a system and method for non-destructivemeasurement of dynamic coercivity effects in magnetic media such thatsubsequent measurements may be made on the magnetic media. The systemand method works in conjunction with standard magnetic media parametrictesters such as spinstand testers so that separate testing devices suchas magnetometers need not be used.

The foregoing description of preferred embodiments of the invention hasbeen presented for the purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed. It is intended that the scope of the invention belimited not by this detailed description, but rather by the claimsappended hereto.

What is claimed is:
 1. A system for non-destructive measurement ofdynamic coercivity of magnetic media, the system comprising: aread/write head for non-destructively writing to and reading frommagnetic media; and means for measuring dynamic coercivity by usingreverse DC erased noise as an indicator for coercivity.